Most types of integrated circuits are manufactured by what is essentially a photographic process. A wafer of semiconductor material of a selected type is coated with a photoresistive material and then a mask is placed over the wafer and subjected to a light source. In the production of integrated circuits, this process may be repeated a number of times using several different photomasks in a desired sequence in order to produce the finished semiconductor chip. Thus, the chip consists of a plurality of layers and patterns produced in this fashion from separate photomasks laid one at a time in precise alignment with one another.
A photomask used for this purpose is a coated glass plate with small multiple photographic images printed thereon which have been built to extremely tight tolerances. Each step in the production of high quality precision photomasks must be carefully controlled for temperature, humidity and extreme cleanliness to insure that dimensional tolerances are met and that defects are kept to a minimum. As will be more precisely described subsequently, the photomasks actually used for the production of a semiconductor chip are the exact size of the finished chip and thus comprise a network of various small lines which are optically opaque separated by small lines and areas which are optically transparent. Obviously, any undesired solid or non-light transmitting particles which might find their way onto these photomasks, either during the use of the photomasks for the production of a chip or during the manufacture of the mask itself, would produce a defective product.
Many other problems can arise, however, during the manufacture of chips. Ordinarily, a photomask comprises an array of die images, each die image representing a separate mask for a separate chip, and the finished chip, as previously stated, may be made up of several layers, each layer being made up from a separate photomask; the die images of which have separate patterns. For example, a typical photomask may comprise several hundred separate die images. When this photomask is applied to a semiconductor wafer, it would be possible to produce from that wafer an equal number of integrated circuits if all of the die images on the photomask were free of defects and if other problems did not create defects during the production process. On the contrary, however, the usual yield for a given wafer may be about ten to thirty percent. After the photographic processes are finished with the wafer, the manufacturer tests each circuit for defects and rejects the defective chips. Due to the complex nature of the devices and the inability to accurately analyze a given chip for the source of the defect which has occurred therein, it is extremely difficult to determine why a particular chip is defective, or at least, it is difficult and expensive to determine the source of the defects. Thus, it is more economical to throw away the rejects, and such is the industry practice. Obviously, one of the most logical sources of a defective chip is a defect in a mask or set of masks.
A great deal of semiconductor material is being wasted under current manufacturing techniques by the low yields being obtained. The general approach by the industry as of the present date towards increasing yield has been a statistical approach. It has not been economically feasible to make a one hundred percent inspection of each finished photomask to ascertain and eliminate all the defects in all of the die images. As aforesaid, a typical mask may comprise several hundred or more die images and to check each die image on the mask may take as long as eight hours or more. Semiconductor manufacturers have not generally been willing to pay for such inspection procedures because they result in a substantial increase in photomask costs. This is, however, one available alternative for the purposes of yield improvement albeit a costly one.
It is currently a typical industry procedure to thoroughly inspect about ten percent of the masks produced for a typical batch, then inspecting ten percent of the area thereof and to allow the mask to pass inspection if it has no more than ten to twenty percent defective die images of those inspected. One way to obtain better yields is for chip manufacturers to increase the inspection techniques and tighten up the standards for rejection. Of course, manufacturers could require one hundred percent inspection of every die image in an array so that the photomask maker must map all defects and then repair as many of the defects as possible in order to make a perfect master plate. Even then, unless the manufacturer is going to use the master plate rather than working plates, there is still a possibility for additional defects. In addition, the laser trimmer technique of repairing defects is only operative where the defect is an opacity such as may be caused by a particle of dust or a void in the photoresistive coating. If the defect is transparent where it should be opaque, there are no presently known commercially usable techniques for repairing such a die image.
Another means of increasing yield would be to use the master plate rather than subplates or working plates made from these subplates. Subplates or submaster plates are made by a contact print from the master and thus the image is reversed. Working plates are contact printed from the subplates or submaster plates. Every reprint from a master plate gives increasing errors either in the image or the fit since size changes are inevitable during the reprinting process. Further, it is obvious that during every reprint there are opportunities for dust particles or voids in the photoresistive coating of the blank plate being printed upon to create further defects.
Another method which might be employed to improve yield is to adjust the design rules for the layout of the integrated circuit such as by changing the geometry of the die image or the pattern within the die image to make more open spaces. While this tends to reduce defects in the die images, it also tends to make them larger, which is contrary to current market demands for smaller size chips. Another technique for reducing defects is to use a proximity or a projection printing process. The currently popular practice is to place the photomask in close physical contact with the silicon wafer and in doing so it is possible to trap opacities between them. Proximity or projection printers do not require that the mask be in contact with the wafer.
There is another technique for very high yield production consisting of a rather expensive and complex device known as an electron beam pattern composition generator which "writes" the circuit pattern on the wafer under computer control. Here, there is nothing at all in contact with the wafer. Such a pattern generator is currently in the research and development stage and presently has a very low production rate.
It will be noted that the commonly used techniques for decreasing defects are primarily techniques that one is to employ as one progresses through the process. Tightening up photomask quality inspection standards and rejection levels and other similar approaches are primarily a statistical attack on the problem. Nothing is known or currently practiced to relate a defective chip with its cause nor is there any known way to relate mask related defects in an exact manner with individual die image failure or sectional wafer failure.